A U.N. spokesman said the country can't unilaterally dissolve the 1953 armistice that ended the Korean War, however

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In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, walks with military personnel as he arrives for a military unit on Mu Islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea.

The Obama administration expressed heightened concern Monday over threats of war from nuclear-armed North Korea and issued new sanctions against the communist nation's primary exchange bank and several senior government officials.

In the last three months, Pyongyang, under its enigmatic young leader Kim Jong Un, has defied the United States and other world powers by testing an intercontinental ballistic missile and testing a third nuclear bomb. Its latest salvo: a declaration carried through state media that it was cancelling the 60-year armistice that maintains peace on the Korean peninsula.

Washington is walking a tight line. It is trying not to overplay the danger from an attention-seeking regime infamous for its bluster but has to take the North's threats of possible military action seriously. The U.S. also is treaty-bound to provide support for South Korea or Japan in the event of any aggression.

"We are certainly concerned by North Korea's bellicose rhetoric," White House spokesman Jay Carney told reporters Monday. "The threats that they have been making follow a pattern designed to raise tension and intimidate others. The DPRK will achieve nothing by threats or provocation, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in northeast Asia."

In an effort to combat North Korea's weapons of mass destruction and ballistic missile programs, the Obama administration issued a series of sanctions Monday.

The Treasury Department designated North Korea's Foreign Trade Bank as a WMD proliferator and imposed similar punitive measures against Paek Se-Bong, the chairman of North Korea's Second Economic Committee, which oversees production of North Korea's ballistic missiles. The designation freezes any assets in the U.S. and bans Americans from doing business with the bank or Paek.

And the State Department placed similar restrictions on three other senior officials: Pak To-Chun, chief of North Korea's weapons production and arms exports department; Chu Kyu-Chang, a munitions industry director; and O Kuk-Ryol, who controls the North's nuclear research and development organization.

But on the immediate threat of an end to the Korean War's armistice agreement, the administration took a more cautious approach.

Speaking at a meeting of the Asia Society in New York, Tom Donilon, President Barack Obama's national security adviser, said the claims may be "hyperbolic." He said the U.S. will protect its allies in any case.

"There should be no doubt: we will draw upon the full range of our capabilities to protect against, and to respond to, the threat posed to us and to our allies by North Korea," Donilon said in remarks prepared for delivery distributed by the White House.

"This includes not only any North Korean use of weapons of mass destruction but also, as the president made clear, their transfer of nuclear weapons or nuclear materials to other states or non-state entities," he said. "Such actions would be considered a grave threat to the United States and our allies and we will hold North Korea fully accountable for the consequences."

Donilon also said Obama will meet with newly inaugurated South Korean President Park Geun-hye at the White House in May.

The South has tried to boost public confidence of by issuing stern warnings of its own. Defense Ministry spokesman Kim Min-seok said last week that North Korea's government would "evaporate from the face of the Earth" if it ever used a nuclear weapon.

Despite North Korea's threat of nuclear strikes on Washington, experts believe Pyongyang still lacks the technology to create a miniaturized warhead to place on a ballistic missile. But with the South Korean capital of Seoul just 40 miles south of the border and the North's million-man army, the risks for the South are more acute.

State Department spokeswoman Victoria Nuland on Monday would only characterize North Korea's statement on pulling out of the Korean War armistice as dangerous rhetoric, and not action. She called the statement concerning but said its impact was unclear.

Besides its vagueness, the U.N. says that North Korea cannot unilaterally dissolve the armistice. And neither South Korea nor the U.S. appears to have received any official communication stating that the cease-fire is no longer in effect.

Even with the armistice, the United States remains technically at war with the notoriously unpredictable North Koreans, whose opaque leadership has confounded successive American administrations. Some 28,500 U.S. troops are stationed in South Korea to deter potential aggression.